A desirable wavelength for laser radar (lidar) and over the horizon optical communications is 1.5 micrometers because this wavelength is considered to be eyesafe. Wavelengths in the range of about 2.8-2.9 micrometers have been found useful for medical applications such as laser surgery because these wavelengths are highly absorbed by the water in tissue and thus are effective for vaporizing tissue. However, reliable, low cost, high power lasers for directly generating such wavelengths are not commercially available.
Pulsed neodymium doped yttrium aluminum garnet (Nd:YAG) lasers are commercially available from a number of suppliers and are used in many fields due to their relatively low cost, large average and peak powers and high pulse repetition frequencies. A well known method for frequency shifting a laser toward longer wavelengths is by stimulated Raman scattering. A laser beam is directed through a Raman cell containing a Raman active medium. When the intensity of the laser beam exceeds a threshold value, light is emitted by the Raman medium at a wavelength that is longer than the wavelength of the laser beam. The output of the Raman cell includes light at the laser wavelength and at the shifted wavelength. The frequency shift and the conversion efficiency are characteristics of the Raman medium.
The use of methane for Raman shifting of a neodymium YAG laser output at 1.06 micrometers to a wavelength of 2.8 micrometers is disclosed by Guntermann et al in Applied Optics, Vol. 28, No. 1, Jan. 1, 1989, pages 135-138. Medical applications are suggested. The generation of 1.54 micrometer radiation for laser radar using methane as a Raman active medium is disclosed by Patterson et al in Applied Optics, Vol. 28, No. 23, Dec. 1, 1989, pages 4978-4981. Deposition of soot like particles on the Raman cell windows was reported to limit the operating life of the system.
Raman scattering using methyl, ethyl and isopropyl alcohol, acetone, trichloroethane and water as Raman active media is disclosed by Colles in Optics Communications, Vol. 1, No. 4, September/October 1969, pages 169-172. Picosecond pulses at 530 nanometers, which were provided by second harmonic generation of the output of a neodymium glass laser, were used as the pump pulses. Stimulated Raman scattering of 100 picosecond pulses in hydrogen, deuterium and methane is disclosed by Hanna et al in IEEE Journal of Quantum Electronics, Vol. QE 22, No. 2, February 1986, pages 332-336. A mode locked and Q-switched neodymium YAG laser followed by a second harmonic generator was used to generate pump pulses at 1.06 micrometers and 0.53 micrometers. The use of hydrogen, deuterium and methane as Raman active media are also disclosed by Lorre et al in IEEE Journal of Quantum Electronics, Vol. QE 15, No. 5, May 1979, pages 337-342 and by Ottusch et al in IEEE Journal of Quantum Electronics, Vol. 24, No. 10, October 1988, pages 2076-2080.
A Raman cell positioned inside a neodymium YAG laser resonant cavity is disclosed in U.S. Pat. No. 4,327,337, issued Apr. 27, 1982 to Liu. Deuterium is suggested as a Raman active medium. A 1.5 micron Raman laser is disclosed in U.S. Pat. No. 3,668,420, issued Jun. 6, 1972 to Vanderslice. A laser system for generating radiation in the ultraviolet wavelength range using a plurality of Raman cells is disclosed in U.S. Pat. No. 4,254,348, issued Mar. 3, 1981 to Stappaerts. Deuterium is disclosed as a Raman active medium.
All the known techniques for generation of radiation at 1.5 micrometers and 2.8-2.9 micrometers have been subject to one or more problems, including a short operating life, low efficiency and Brillouin backscattering from the Raman active medium. It is desirable to provide laser systems which overcome these problems.
It is a general object of the present invention to provide improved laser systems.
It is another object of the present invention to provide laser systems for efficient Raman shifting of the 1.06 micrometer radiation from a neodymium laser to about 1.5 micrometers.
It is a further object of the present invention to provide laser systems for efficient Raman shifting of the 1.06 micrometer radiation from a neodymium laser to about 2.8-2.9 micrometers.
It is another object of the present invention to provide reliable, long life laser systems for generating radiation at about 1.5 micrometers and about 2.8-2.9 micrometers.
It is yet another object of the present invention to provide Raman active media for efficient conversion of 1.06 micrometer radiation to radiation at about 1.5 micrometers and about 2.8-2.9 micrometers.